Apparatus and method for the compression of hydrogen gas

ABSTRACT

An electrochemical hydrogen compression apparatus and a method for the collection and compression of hydrogen. The apparatus includes a cathode having a surface that is modified with a Tafel inhibitor that inhibits the Tafel reaction at the cathode surface. The same cathode surface may also include a Volmer catalyst to enhance the Volmer reaction at the cathode surface. The apparatus and method can advantageously increase the pressure of collected hydrogen, and may be useful for supplying the hydrogen to energy producing devices, such as a fuel cell.

FIELD

This disclosure relates to the field of hydrogen compression, particularly hydrogen compression using an electrochemical cell having an electrode surface modification.

BACKGROUND

Hydrogen is currently used as an alternative energy source, and it is projected that the use of hydrogen will continue to grow due to the demand for clean and reliable energy sources. These applications often require a source of high-purity hydrogen that is under high pressure. However, feasible methods to produce hydrogen of high purity and under high pressure remain a challenge.

One method of hydrogen storage that is of interest is electrochemical storage of hydrogen. This method stores a portion of the hydrogen generated by water electrolysis, for example, within the cathode of an electrochemical cell. Examples of such methods and electrochemical cells are illustrated in U.S. Pat. No. 7,955,491 by Ballantine et al. and U.S. Patent Publication No. 2004/0211679 by Wong et al., each of which is incorporated herein by reference in its entirety.

However, there is a need for further improvements in such methods and devices for the electrochemical storage of high-purity hydrogen.

SUMMARY

In accordance with the present disclosure, it has been found that a surface modification of the cathode in an electrochemical hydrogen compression apparatus may advantageously increase the obtainable hydrogen pressure. In certain characterizations, the obtainable hydrogen pressure may be such that only a single compression stage is required to achieve a pressure that is often needed for delivery of the hydrogen, such as at least about 600 bar (60 MPa).

Unlike mechanical compressors, there are no moving parts that can wear out. Suitable cathode alloys (e.g. Pd—Ag) may be employed to mitigate stress fractures and solution hardening by eliminating or reducing the phase changes associated with hydrogen absorption.

In one embodiment, an apparatus for the compression of hydrogen gas is disclosed. The apparatus includes an anode, a palladium cathode, and an electrolytic medium separating the anode and the palladium cathode. The palladium cathode comprises a first palladium surface in operative contact with the electrolytic medium and a second palladium surface separated from the first palladium surface by a cathode thickness. The first palladium surface comprises a Tafel inhibitor that increases the hydrogen adsorption energy of the first palladium surface.

The apparatus of this embodiment may be subject to various refinements and characterizations, alone or in any combination. For example, in one characterization, the Tafel inhibitor increases the hydrogen adsorption energy of the first palladium surface by at least about 0.3 eV for 0.5 monolayer of hydrogen surface coverage. In a further refinement, the Tafel inhibitor increases the hydrogen adsorption energy of the first palladium surface by at least about 0.8 eV for 0.5 monolayer of hydrogen surface coverage.

In another characterization, Tafel inhibitor is selected from the group consisting of lead, bismuth, mercury, indium, zinc, thallium, cadmium and combinations thereof. In a further refinement, the Tafel inhibitor is selected from the group consisting of lead, bismuth and combinations thereof.

In another characterization, the second palladium surface has a concentration of the Tafel inhibitor of less than about 1.0 monolayer. In a further refinement, the second palladium surface is substantially free of the Tafel inhibitor.

In another characterization, the concentration of the Tafel inhibitor on the first palladium surface is at least about 0.5 μg/cm². In a further refinement, the concentration of the Tafel inhibitor on the first palladium surface is at least about 1.0 μg/cm². In het e further refinement, the concentration of the Tafel inhibitor on the first palladium surface is at least about 2.0 μg/cm².

In another characterization, the concentration of the Tafel inhibitor on the first palladium surface is not greater than about 20 μg/cm². In one refinement, the concentration of the Tafel inhibitor on the first palladium surface is not greater than about 10 μg/cm². In yet a further refinement, the concentration of the Tafel inhibitor on the first palladium surface is not greater than about 5.0 μg/cm².

In another characterization, the Tafel inhibitor is in the form of a layer deposited on the first palladium surface. In one refinement, the deposited layer has a thickness of not greater than about 0.1 μm. In yet a further refinement, the deposited layer has a thickness of not greater than one monolayer.

In another characterization, the first palladium surface further comprises a Volmer catalyst. In one refinement, the Volmer catalyst is selected from the group consisting of platinum, iridium, rhodium, ruthenium, rhenium, and combinations thereof. In a further refinement, the Volmer catalyst comprises a metal selected from platinum, iridium and combinations thereof. In another refinement, the ratio (e.g., atomic ratio) of the Volmer catalyst to the Tafel inhibitor on the first palladium surface is not greater than about 5. In a further refinement, the ratio of the Volmer catalyst to the Tafel inhibitor on the first palladium surface is not greater than about 0.5. In a further refinement, the ratio of the of the Volmer catalyst to the Tafel inhibitor on the first palladium surface is at least about 0.01. In yet a further refinement, the ratio of the of the Volmer catalyst to the Tafel inhibitor on the first palladium surface is at least about 0.1.

In another characterization, the second palladium surface comprises a Volmer catalyst. In one refinement, the Volmer catalyst on the second palladium surface is selected from the group consisting of platinum, iridium, rhodium, ruthenium, rhenium, and combinations thereof.

In another characterization, the cathode thickness is at least about 5 μm. In another characterization, the cathode thickness is not greater than about 5 cm.

In yet another characterization, the electrolytic medium comprises an acidic electrolyte. In another characterization, the electrolytic medium comprises an alkaline electrolyte. In yet another characterization, the electrolytic medium comprises an anion conducting electrolyte. In another characterization, the electrolytic medium comprises a proton conducting electrolyte.

In yet a further characterization, the electrolytic medium comprises a membrane disposed between the anode and the cathode. In one refinement, the membrane bounds a cathode compartment comprising the cathode and an anode compartment comprising the anode. In a further refinement, the membrane is an anion exchange membrane. In yet a further refinement, the cathode compartment contains a first electrolyte and the anode compartment contains a second electrolyte, where each of the first and second electrolytes are alkaline electrolytes. In another refinement, the membrane is a proton exchange membrane. In a further refinement, the cathode compartment contains a first electrolyte and the anode compartment contains a second electrolyte, where each of the first and second electrolytes are acidic electrolytes. In another refinement, the cathode compartment contains a first electrolyte and the anode compartment contains a second electrolyte, where the first electrolyte is an alkaline electrolyte and the second electrolyte is an acidic electrolyte.

In another characterization of the apparatus, the anode comprises platinum. In another characterization, the apparatus includes a hydrogen source that is operatively connected to the anode to supply hydrogen to the anode. In another characterization, the apparatus includes a power source operatively connected to the anode and to the cathode.

In another characterization, the apparatus is a substantially cylindrical cell assembly, wherein the anode is a substantially cylindrical structure that is centrally disposed within the substantially cylindrical cell assembly, and the cathode surrounds the anode. In another characterization, the apparatus includes a gas containment structure surrounding the cathode and configured to contain hydrogen gas between the cathode and gas containment structure.

In another embodiment, a method for the compression of hydrogen gas is disclosed. The method includes applying an electrical potential between an anode and a palladium cathode that is separated from the anode by an electrolytic medium. The electrical potential causes hydrogen species to form near a first palladium surface of the cathode that is in operative contact with the electrolytic medium. Hydrogen is collected in an enclosed volume that is at least partially bounded by a second palladium surface of the cathode that is separated from the first palladium surface by a cathode thickness. The first palladium surface of the cathode comprises a first Tafel inhibitor that increases the hydrogen adsorption energy of the first palladium surface by at least about 0.3 eV for 0.5 monolayer of hydrogen surface coverage.

The method of this embodiment may be carried out using the apparatus disclosed herein, including any combination of the characterizations and refinements discussed above. In one characterization of this method, the first Tafel inhibitor increases the hydrogen adsorption energy of the first palladium surface by at least about 0.8 eV for 0.5 monolayer of hydrogen surface coverage. In another characterization, the first Tafel inhibitor is selected from the group consisting of lead, bismuth, mercury, indium, zinc, thallium, cadmium and combinations thereof. In one refinement, the first Tafel inhibitor is selected from the group consisting of lead, bismuth and combinations thereof.

In another characterization, the first palladium surface of the cathode has an adsorbed hydrogen chemical potential of greater than about 0.05 eV. In yet another characterization, the first palladium surface of the cathode has an adsorbed hydrogen chemical potential of greater than about 0.120 eV. In yet another characterization, the step of collecting hydrogen comprises containing the hydrogen in the enclosed volume at a pressure of at least about 600 bar. In one refinement, the step of collecting hydrogen comprises containing the hydrogen in the enclosed volume at a pressure of at least about 800 bar.

In another characterization, the second palladium surface of the cathode comprises a PdH_(x) alloy, where x is greater than about 0.8. In a further refinement, the second palladium surface of the cathode comprises a PdH_(x) alloy, where x is greater than about 0.9. In another characterization, the collected hydrogen has a purity of at least about 98%.

Additional characterizations and refinements of the foregoing apparatus and method will become apparent to those of skill in the art based on the following disclosure.

DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an embodiment of a hydrogen compression apparatus according to the present disclosure.

FIG. 2 schematically illustrates an embodiment of an electrode assembly according to the present disclosure.

FIG. 3 illustrate another embodiment of an electrode assembly according to the present disclosure.

FIGS. 4A and 4B illustrate another embodiment of a hydrogen compression apparatus according to the present disclosure.

FIGS. 5A-5D are Tafel plots illustrating the efficacy of certain Tafel inhibitors according to the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

The present disclosure relates to an apparatus and method for the compression of hydrogen. In certain aspects, the apparatus and method may also be used to produce and/or deliver compressed hydrogen of very high purity.

In one embodiment, an apparatus for the compression of hydrogen (e.g., the compression of hydrogen gas) is disclosed. The apparatus comprises an electrochemical cell having an anode and a cathode that are separated by an electrolytic medium. The cathode is configured to enhance the compression of hydrogen through the implementation of a modified cathode surface that enhances (e.g., increases) the hydrogen overpotential at the surface.

FIG. 1 schematically illustrates an apparatus for the compression of hydrogen according to an embodiment of the present disclosure. The apparatus 100 includes an electrode assembly 102 having an anode 110, a cathode 120 and an electrolytic medium 140 separating (e.g., disposed between) the anode 110 and the cathode 120. A power source 160 is operatively connected to the electrode assembly 102, e.g., to apply an electrical potential between the anode 110 and the cathode 120.

In the embodiment illustrated in FIG. 1, water in the electrolytic medium 140 is dissociated via electrolysis to form H⁺ and OH⁻:

Anode: 2H₂O↔O₂+4H⁺4e ⁻  [1]

Cathode: 2H⁺+2e ⁻↔H₂  [2]

The hydrogen contacts a first surface 122 of the cathode and diffuses through the cathode 120 to a second surface 124 and is collected in a gas containment structure 170 that is at least partially defined by the second surface 124 of the cathode 120. A valve 172 or similar structure may be provided to selectively remove hydrogen from the containment structure 170.

Hydrogen evolution via electrolysis in an alkaline electrolyte by way of Equation [2] and hydrogen absorption at the cathode surface 122 are believed to occur through a series of reactions, namely the following four reaction steps:

H₂O+M+e ⁻↔MH_(surf)+OH⁻  [3]

2MH_(surf)↔2M+H₂  [4]

H₂O+MH_(surf) +e ⁻↔H₂+OH⁻+M  [5]

MH_(surf)↔M_(b)H_(bulk)  [6]

The foregoing reactions are referred to as: [3] the Volmer reaction; [4] the Tafel reaction; [5] the Heyrovsky reaction; and [6] the Penetration reaction. M, M_(b), H_(surf), and H_(bulk) represent the adsorption site on the surface metal, the absorption site in the bulk metal, adsorbed hydrogen, and absorbed H, respectively. For hydrogen storage, it is desirable to increase the concentration of M_(b)H_(bulk), e.g., to increase the concentration of hydrogen in the cathode 120. In accordance with the present disclosure, the concentration of M_(b)H_(bulk) may be enhanced by suppressing the generation of H₂ via the Tafel reaction at the cathode surface 122.

FIG. 2 schematically illustrates a cross-sectional view of an electrode assembly 202 that includes an anode 210, a cathode 220 and an electrolytic medium 240 separating the anode 210 and the cathode 220. The bulk of the cathode 220 may comprise a noble metal, and in one characterization the cathode comprises palladium (Pd), as palladium is a suitable material for the selective diffusion of hydrogen through the cathode. 220. As used herein, the term “palladium cathode” refers to a palladium content of at least about 50%. The palladium may be alloyed with one or more alloying elements, such as silver, gold, copper, nickel, iron, yttrium, cerium and vanadium. For example, the palladium cathode 220 may comprise a palladium alloy comprising at least about 50% palladium, such as at least about 60% palladium, such as at least about 70% palladium. In one characterization, the palladium is alloyed with silver. One particularly useful alloy comprises about 77% palladium and about 23% silver. In other embodiments, the cathode may comprise at least about 98% palladium, at least about 99% palladium, or even at least about 99.5% palladium. The palladium cathode 220 may be porous or non-porous, and in one embodiment, the cathode 220 comprises substantially non-porous palladium, e.g., a non-porous palladium sheet.

In accordance with the present disclosure, the first surface 222 of the cathode (e.g., the surface in operative contact with the electrolytic medium 240) includes at least a first Tafel inhibitor, e.g., that is deposited onto the palladium. The Tafel inhibitor is selected to inhibit the Tafel reaction at the cathode surface 222, e.g., to inhibit the formation of H₂ at the surface. Stated another way, the first Tafel inhibitor is selected to enhance the ability of hydrogen to diffuse from the first surface 222 into the bulk of the cathode 220. Stated yet another way, the first Tafel inhibitor 226 is selected to increase the hydrogen adsorption energy of the first surface 226 as compared to the same surface that has not been modified with the Tafel inhibitor.

The efficacy of the Tafel inhibitor may be characterized in a number of ways. In one characterization, the first Tafel inhibitor increases the hydrogen adsorption energy of the first cathode surface 222 by at least about 0.3 eV for a 0.5 monolayer of hydrogen surface coverage. That is, when the first Tafel inhibitor is provided (e.g., deposited) on the first cathode surface 222, the hydrogen adsorption energy at a hydrogen coverage of about 0.5 monolayer on the first cathode surface 222 will increase by at least about 0.3 eV. In another characterization, the hydrogen adsorption energy of the first cathode surface 222 increases by at least about 0.5 eV for 0.5 monolayer of hydrogen surface coverage, and in a further characterization the hydrogen adsorption energy of the first cathode surface increases by at least about 0.8 eV for 0.5 monolayer of hydrogen surface coverage.

To obtain such an increase in the hydrogen adsorption energy, the Tafel inhibitor may include a metallic element that is selected from the group consisting of lead, bismuth, mercury, indium, zinc, thallium, cadmium and combinations thereof. In certain characterizations, the Tafel inhibitor includes a metallic element selected from lead, bismuth or a combination of lead and bismuth. Lead and bismuth are particular effective as Tafel inhibitors for the purposes of the present disclosure.

The Tafel inhibitor may be deposited onto the first (inner) surface 222 of the cathode 220 in relatively low concentrations, expressed as the mass of Tafel inhibitor per unit area of the cathode surface 222. In one characterization, the concentration of the Tafel inhibitor on the first surface of the cathode may be at least about 0.5 μg/cm², such as at least about 1.0 μg/cm², such as at least about 2.0 μg/cm². In a further characterization, the concentration of the first Tafel inhibitor on the first surface of the cathode may be not greater than about 20 μg/cm², such as not greater than about 10 μg/cm², or even not greater than about 5.0 μg/cm². Thus, in certain characterizations the concentration of the first Tafel inhibitor on the first surface 222 of the cathode 220 may be from about 0.5 μg/cm² to about 20 μg/cm², such as from about 1.0 μg/cm² to about 10 μg/cm², such as from about 2.0 μg/cm² to about 5.0 μg/cm².

Characterized in another way, the Tafel inhibitor may be in the form of a material layer on the first surface 222 of the cathode 220. In this characterization, the material layer may have a thickness of not greater than about 100 nm, such as not greater than about 75 nm, or not greater than about 50 nm. In another characterization, the “thickness” of the material layer of the Tafel inhibitor may be expressed in terms of a monolayer of coverage. In certain characterizations, the thickness may be not greater than about one monolayer, such as not greater than about 0.75 monolayer.

Thus, the Tafel inhibitor resides on the first surface 222 of the cathode. The second (e.g. outer) surface 224 of the cathode may be characterized as being substantially free of the Tafel inhibitor. The second surface 224 is separated from the first surface 222 by a cathode thickness (t_(c)), i.e., the thickness of the palladium layer. The cathode thickness may be characterized as having a thickness of at least about 5 μm, such as at least about 20 μm, or even at least about 100 μm. In another characterization, the cathode thickness may be not greater than about 5 cm, such as not greater than about 1 cm and even not greater than about 0.1 cm. It will be appreciated that the apparatus may include a supporting structure for the cathode, e.g., a porous supporting structure, such as when the palladium cathode is very thin.

The electrolytic medium 240 separates the anode 210 and the cathode 220, and may comprise a single component (e.g., an alkaline electrolyte) or may comprise multiple components, including a selective membrane. In one characterization, the electrolytic medium includes an alkaline electrolyte. Examples of useful alkaline electrolytes include, but are not limited to, strong bases such as lithium hydroxide (LiOH), potassium hydroxide (KOH), calcium hydroxide (CaOH), sodium hydroxide (NaOH) and the like. Alkaline electrolytes may be selected for their ability to conduct anions through the electrolyte. Alternatively, the electrolytic medium may comprise an acidic electrolyte including, but not limited to, sulfuric acid (H₂SO₄), nitric acid (HNO₃), hydrochloric acid (HCl), perchloric acid (HClO₄) and the like. The electrolytic medium may also include a selective membrane, such a proton conducting membrane or an anion conducting membrane. The membrane may bound an anode compartment comprising the anode, and a cathode compartment between the cathode and the membrane. The anode compartment and the cathode compartment may contain the same type of electrolyte, or may contain different types of electrolytes.

For example, in one embodiment, the electrolytic medium comprises an anion exchange membrane bounding a cathode compartment comprising the cathode and an anode compartment comprising the anode. The cathode compartment may comprise a first alkaline electrolyte and the anode compartment may comprise a second alkaline electrolyte. The first and second alkaline electrolytes may be the same electrolyte or may be different electrolytes. In another embodiment, the electrolytic medium comprises a proton exchange membrane. In this embodiment, the cathode compartment may comprise a first acidic electrolyte and the anode compartment may comprise a second acidic electrolyte. The first and second acidic electrolytes may be the same electrolyte or may be different electrolytes. In a further embodiment, membrane may separate a cathode compartment comprising the cathode and an anode compartment comprising the anode, where the anode compartment and cathode compartment comprise a different type of electrolyte. For example, the electrolyte in the anode compartment may be an acidic electrolyte and the electrolyte in the cathode compartment may be an alkaline electrolyte. For example, in one embodiment, the electrolytic medium on the anode side of the member is sulfuric acid, the proton exchange membrane separating the cathode and anode compartments is a sulfonated tetrafluoroethylene based fluoropolymer membrane (e.g., NAFION), and the cathode compartment contains a lithium hydroxide electrolyte.

FIG. 3 schematically illustrates a cross-sectional view of an electrode assembly 302 that also includes an anode 310, a cathode 320 and an electrolytic medium 340 separating the anode 310 and the cathode 320. The cathode 320 may be as characterized in FIG. 2 above, e.g., primarily comprising palladium to facilitate the selective diffusion of hydrogen through the cathode 320.

As illustrated in FIG. 3, the first surface 322 of the cathode 324 comprises a Volmer catalyst in addition to the Tafel inhibitor. The Volmer catalyst is selected to enhance (e.g., catalyze) the Volmer reaction above, e.g., to enhance the absorption of hydrogen on the cathode surface 322 (MH_(surf)). Effective Volmer catalysts may be selected from the group consisting of platinum, iridium, rhodium, ruthenium, rhenium, and combinations thereof. Particularly useful Volmer catalysts may be selected from platinum, iridium and combinations thereof.

The ratio of the Volmer catalyst to the Tafel inhibitor on the first cathode surface 322 may be not greater than about 5, such as not greater than about 1, such as not greater than about 0.5. In another characterization, the ratio of Volmer catalyst to Tafel inhibitor on the first cathode surface 322 may be at least about 0.01, such as at least about 0.1. Thus, in one characterization, the ratio of Volmer catalyst to Tafel inhibitor on the first cathode surface is from about 0.1 to about 0.5.

In certain characterizations, the second (outer) surface 324 of the cathode 320 may also comprise a Volmer catalyst. A Volmer catalyst may be implemented on the second surface 324 either with or without the use of a Volmer catalyst on the first surface 322. Further, the Volmer catalyst on the second surface 324 may be the same or may be different than the Volmer catalyst used on the first surface 322.

When a Volmer catalyst is applied to the second surface 324, the concentration of the Volmer catalyst may be at least about 0.1 μg/cm², such as at least about 0.3 μg/cm². In another characterization, the concentration of the Volmer catalyst may be not greater than about 50 μg/cm², such as not greater than about 30 μg/cm², or not greater than about 15 μg/cm².

Referring back to FIG. 1, the hydrogen compression apparatus 100 includes a power source 160, e.g., a DC power source. The power source 160 is configured to apply an electrical potential to the electrode assembly 102, i.e., between the anode 110 and the cathode 120.

FIGS. 4A and 4B schematically illustrate another embodiment of an apparatus for the compression of hydrogen according to the present disclosure, with FIG. 4A illustrating a side view and FIG. 4B illustrating a top view. The apparatus 400 is a substantially cylindrical assembly of the various components, where the various components are concentrically disposed relative to one another to form the cylindrical hydrogen compression apparatus 400. As illustrated in FIGS. 4A and 4B, an anode 410 is centrally disposed within the apparatus 400. An electrolytic medium 440 surrounds anode to separate the anode 410 from the cathode 420. As illustrated in FIGS. 4A and 4B, the membrane electrode assembly 440 includes a selective membrane 446 separating a first electrolyte 442 and a second electrolyte 444. A power supply 460 provides an electrical potential between the anode 410 and the cathode 420.

The cathode 420 includes an inner surface 422 facing the anode 410 that includes at least a Tafel inhibitor as is described above with respect to FIGS. 2 and 3. On the opposite side of the cathode 420 is a gas containment structure 470 that is configured to store hydrogen under high pressure. A valve 472 or similar structure may be provided to selectively release hydrogen from the containment structure 470.

The present disclosure is also directed to a method for the compression of hydrogen, e.g., of hydrogen gas. The method includes applying an electrical potential between an anode and a cathode, e.g., a palladium cathode that is separated from the anode by an electrolytic medium, to cause hydrogen species to form near a first surface of the cathode that is in operative contact with the electrolytic medium. Hydrogen is collected in an enclosed volume that is at least partially bounded by the cathode, e.g., by a second surface of the cathode that is separated from the first cathode surface by a cathode thickness. The first cathode surface comprises a Tafel inhibitor that increases the hydrogen adsorption energy of the first surface by at least about 0.3 eV for a 0.5 monolayer of hydrogen surface coverage.

Although not limited to any particular apparatus, the method may be carried out using an apparatus as described above, e.g., with respect to FIGS. 1-4. As is discussed above in relation to the hydrogen compression apparatus, the Tafel inhibitor may advantageously increase the hydrogen adsorption energy of the first palladium surface by at least about 0.8 eV for 0.5 monolayer of hydrogen surface coverage. The Tafel inhibitor may be selected from the group consisting of lead, bismuth, mercury, indium, zinc, thallium, cadmium and combinations thereof, and in one characterization may be selected from the group consisting of lead, bismuth and combinations thereof.

A relatively high adsorbed hydrogen chemical potential is indicative of a high concentration of hydrogen in the cathode, e.g., in the palladium. In one characterization, the method results in the first surface of the cathode having an adsorbed hydrogen chemical potential of greater than about 0.05 eV, such as greater than about 0.120 eV.

The method is also capable of collecting the hydrogen and storing the hydrogen at high pressures, without the need for mechanical pumps or similar devices. For example, the step of collecting the hydrogen may include containing the hydrogen in the enclosed volume at a pressure of at least about 600 bar, such as at a pressure of at least 700 bar, or even a pressure of at least 800 bar.

The ability to store hydrogen under elevated pressure in an electrochemical hydrogen apparatus is also related to the ability of the cathode to absorb hydrogen, e.g., as an alloy PdH_(x). For example, the method may be carried out such that the second surface of the cathode comprises a PdH_(x) alloy, where x is at least about 0.80, where x is at least about 0.85, or even where x is at least about 0.90. The method may also result in the collection of hydrogen having a very high purity. In one characterization, the collected hydrogen has a purity of at least about 95%, such as at least about 98%, or even at least about 99%.

In one embodiment, power for hydrogen compression and/or storage is supplied to the hydrogen compression apparatus and/or to the hydrogen compression method using a renewable and/or alternative energy system, e.g., an energy system that does not rely upon the use of fossil fuels. For example, the apparatus of the present disclosure may be scaled to meet the power management needs of a stand-alone wind, photovoltaic and/or fuel cell energy system. The excess energy generated by the energy system during peak production (e.g., under steady winds or under full sun) may be stored by the hydrogen compression apparatus in the form of a chemical potential that can be released as demand exceeds real-time supply. The byproduct of the release of this stored energy is simply water.

Other applications of the disclosed apparatus and method include a chemical potential driven hydrogen compressor to meet the hydrogen storage and delivery requirements for transportation applications, such as for fuel cell driven vehicles. Such storage and delivery requirements may advantageously be met by a single stage compressor.

Examples

The effects of Bi, Pt, Pb, and Bi+Pt deposition on the cathode surface are studied. A measurement of the level of occlusion of H by Pd in the cathode is an indicator of the ability of the cathode to store H under high pressure.

Cathode Assembly

Palladium (Pd) cathodes are configured to allow simultaneous four-point resistance measurements during the electrochemical experiments. Pd foils (ESPI, 99.95% Pd, ESPI Metals, Ashland Oreg., USA) are cold-rolled to a mirror finish, a thickness of from about 40 μm to 50 μm, and are cut to dimensions of about 7 mm×40 mm. Five platinum (Pt) wires are spot welded directly to the Pd foils using a tungsten tip. The Pt wires are encapsulated using PTFE/FEP shrink tubing (Zeus Industrial Products, Inc., Orangeburg, S.C., USA). The contacts to the foil are entombed with polydimethylsiloxane (PDMS, Sylgard® 184, Dow Corning Corp., Auburn, Mich., USA) inside PTFE/FEP shrink wrap tubes. The assembly is placed in an oven at 100° C. for at least about 1 hour to cure the PDMS. The average geometric surface area of the cathodes exposed to the electrolyte is about 4.6±0.1 cm². All reported current densities (j) will be referred to the geometric area.

Configuration of the Electrochemical Cell

A borosilicate glass split cell is used where a sulfonated tetrafluoroethylene based fluoropolymer proton exchange membrane (NAFION 1110, Chemours, Wilmington, Del., USA) separates the Pd cathode and the Pt anode compartments to prevent deposition of Pt from the anode onto the cathode. Argon (Ar) purging is performed in both compartments throughout the experiments to mitigate the effects of O₂ on the measurements. The Pd cathode and a Gaskatel® reversible hydrogen reference electrode (RHE, Gaskatel Gesellschaft für Gassysteme durch Katalyse and Elektrochemie mbH, Kassel, DE) are placed in 0.1 M LiOH, and the Pt foil anode is placed in 0.1 M H₂SO₄.

Energy Dispersive X-Ray Spectroscopy (EDS) Measurements

EDS measurements are taken using a scanning electron microscope equipped with a Bruker XFlash 410-M X-ray detector. The electron energy is 5 kV and the beam intensity is set to 19.

Electrochemical Measurement Procedure

Electrochemical measurements are performed under computer control using a 110 Bio-Logic VSP potentiostat/galvanostat in galvanostatic (constant current) mode. The galvanostatic step cycle consists of 11 steps from −0.5 mA to −50.5 mA in −5 mA increments. The current is held for 2 hours at each current setting to allow time for the hydrogen content in the cathode to reach its quasi-steady state value. The stability of the surface layers is studied by observing the changes in the stored hydrogen by running through at least three galvanostatic step cycles. To eliminate effects associated with solution hardening, the cathode is loaded and then fully deloaded at least twice. The second loading cycle is used for the analysis.

Deposition Procedures

The electrochemical measurement procedure is first performed on all the Pd cathodes in their pristine state without a coating to serve as controls. This also solution hardens and deforms the cathodes so that these effects are separated from the surface treatments. Before performing a deposition, the samples are run anodically in 1% tr. metal grade HNO₃ up to 0.3 V versus Pd until the resistance is within about 1% of the original resistance, indicating that there is less than 1 at. % H left in the Pd. Underpotential deposition (UPD) of the materials is selected as the primary deposition method, as it is expected to deposit uniformly and to 1 monolayer (ML).

Bi Surface Deposition

Bi is deposited onto the Pd cathode surface by UPD, and the materials layer of Bi is assumed to be about 1 monolayer thick. The Bi is deposited from a Bi₂(SO₄)₃ solution prepared by dissolving Bi (99.998% Metal Shipper, West Chester, Pa., USA) in hot concentrated H₂SO₄ (trace metal grade, Fischer Scientific, Waltham, Mass., USA). The concentrated H₂SO₄ solution is diluted to 1 mM Bi in 0.5 M H₂SO₄. The UPD is performed by first holding the sample at 0.9 V vs. Ag/AgCl for 1 minute prior to stepping the potential down to 0.1 V vs. Ag/AgCl for 5 min. UPD at 0.1 V vs. Ag/AgCl is chosen because cyclic voltammetry measurements show bulk Bi deposition occurs at roughly 0.04 V vs. Ag/AgCl. EDS analysis for this sample shows 1.8±0.5 at. % Bi at 5 kV.

Bi Bulk Deposition

The second investigated Bi surface is performed by bulk deposition of Bi at −0.5 V vs. Ag/AgCl for 3 minutes. This potential is below that required for electrolysis, so it is likely that the coating does not have the same uniformity as that of the Bi surface deposition sample. EDS shows 4.9±0.9 at. % Bi, roughly three times the amount of Bi obtained by the UPD of Bi.

Pb Surface Deposition

UPD of Pb is performed using a solution of 0.1 M NaClO₄, 1.0 mM HClO₄, and 1.5 mM Pb(ClO₄)₂ at a potential of −0.425 V vs. Ag/AgCl, held for 4 min.

Bi+Pt Surface Deposition

Pt is deposited via a galvanic exchange reaction between Pt and Bi. After Bi deposition, the sample is rinsed in deionized water and placed in a solution of 0.1 M NaClO₄, 5 mM HClO₄, and 5 mM K₂PtCl₄ for about 20 minutes. EDS measurements show a final Pt/Bi atomic ratio of about 0.13±0.01 after the Bi/Pt exchange (Bi: 2.4±0.4 at %, Pt: 0.32±0.04 at % at 5 kV).

Pt Deposition

Pt is deposited by galvanic exchange between Pt and H. The potentiostatic H stripping is not performed after running a Bi UPD sample, which allowed H to remain on the surface to be available for the Pt/H exchange. This results in significant Pt deposition during the 12 minute exposure to the Pt solution. EDS analysis shows about 40-50 at. % Pt at 5 kV. Thus, this sample is used as a surrogate to study the effects of a bulk Pt surface. In contrast, soaking a piece of pristine Pd overnight in the Pt solution results in an EDS measured Pt amount of <0.1 at. % at 5 kV, suggesting that the rate of exchange between Pt and Pd is insignificant.

Procedure to Determine H Content

The four-point resistance measurement is used to infer the H content in PdH_(x). This technique uses the empirical relationship between the atomic H/Pd ratio (x) and the ratio of the measured resistance (R) to the initial resistance (R_(o)) by fitting experimental data to the fourth order polynomial equation:

R/Ro=1+1.69731x−5.34162x ²+13.4472x ³−9.87644x ⁴  [7]

The resistance is measured using an HP4263B LCR meter with 1 Vrms and 10 kHz signal. The LCR meter is isolated from the galvanostat via a transformer circuit.

The effects of changing the elemental composition of the Pd cathode surface to include Pt, Bi, Bi/Pt, and Pb as described above are investigated by analyzing the traditional Tafel plot (x vs Log(|j|)), x vs η, and corresponding η₂ vs η plots, where η is the measured overpotential and η₂ is the chemical potential of the adsorbed H. The plots for a Pd control sample, a Bi Tafel inhibitor, Pt, and a Bi/Pt sample are illustrated in FIGS. 5A to 5D.

FIG. 5A shows that the Pt surrogate did not have a significant impact on hydrogen absorption (x). By analyzing the Tafel plot in FIG. 5B, the addition of Pt required even lower overpotentials than the Pd reference sample. This result corroborates previous reports showing Pt to have enhanced HER activity over Pd in alkaline solutions. The Pt overlayer improved the hydrogen absorption at an equivalent overpotential (n), as is illustrated by FIG. 5C. A shift in the η₂ vs n plot (FIG. 5D) to smaller cathodic overpotentials is a result of larger j_(oV)/j_(oT). This means that at any selected overpotential, the Pt overlayer would result in the maximum x as compared to all investigated surfaces, assuming it stays on a linear trajectory. However, Pt is such a good catalyst that extraordinary currents are required to obtain a high enough overpotential to obtain the higher hydrogen absorption.

Next, the addition of Bi to the Pd surface showed an increase in hydrogen adsorption of up to 7% of the Pd control (FIG. 5A). In addition, the poor HER activity of Bi causes an increase in the cathodic overpotentials necessary to drive the investigated currents (FIG. 5B). At larger cathodic overpotentials of −η>0.7 V, a curve in the x vs η and x vs Log(|j|) plots is observed and is indicative of the Heyrovsky reaction overtaking the Tafel reaction. Finally, the addition of Bi requires larger cathodic overpotentials than the other investigated samples to achieve an equivalent hydrogen absorption, resulting in a shift in the linear portion of the η₂ vs η curve to larger cathodic overpotentials.

The Bi+Pt surface shows the most improvement in the amount of electrochemically stored H, with up to 13% greater adsorbed hydrogen than the Pd control. (FIG. 5A). For comparison, a H₂ fugacity exceeding 2400 atm is required to obtain the equivalent PdH_(0.87) by H₂ gas loading. The addition of a small amount of Pt to the Bi layer reduces the cathodic overpotentials compared to the Bi layer alone, but still requires larger cathodic overpotentials than the Pd control. The η₂ vs η curve shifts towards lower cathodic overpotentials, again indicating an increase in j_(oV)/j_(oT). This sample outperformed the maximum x for all samples, indicating a successful engineering of a surface to enhance hydrogen adsorption by the combination of a Tafel inhibitor (Bi) and a Volmer catalyst (Pt).

While various embodiments of an apparatus and method for the compression of hydrogen have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present disclosure, including, but not limited to, the following claims. 

What is claimed is:
 1. An apparatus for the compression of hydrogen gas, comprising: an anode; a palladium cathode; and an electrolytic medium separating the anode and the palladium cathode, wherein the palladium cathode comprises a first palladium surface in operative contact with the electrolytic medium and a second palladium surface separated from the first palladium surface by a cathode thickness, wherein the first palladium surface comprises a Tafel inhibitor that increases the hydrogen adsorption energy of the first palladium surface.
 2. The apparatus recited in claim 1, wherein the Tafel inhibitor increases the hydrogen adsorption energy of the first palladium surface by at least about 0.3 eV for 0.5 monolayer of hydrogen surface coverage.
 3. The apparatus recited in claim 1, wherein the Tafel inhibitor is selected from the group consisting of lead, bismuth, mercury, indium, zinc, thallium, cadmium and combinations thereof.
 4. The apparatus recited in claim 3, wherein the Tafel inhibitor is selected from the group consisting of lead, bismuth and combinations thereof.
 5. The apparatus recited in claim 1, wherein the second palladium surface is substantially free of the Tafel inhibitor.
 6. The apparatus recited in claim 1, wherein the concentration of the Tafel inhibitor on the first palladium surface is at least about 0.5 μg/cm².
 7. The apparatus recited in claim 1, wherein the concentration of the Tafel inhibitor on the first palladium surface is not greater than about 10 μg/cm².
 8. The apparatus recited in claim 1, wherein the Tafel inhibitor is in the form of a layer deposited on the first palladium surface.
 9. The apparatus recited in claim 8, wherein the deposited layer has a thickness of not greater than one monolayer.
 10. The apparatus recited in claim 1, wherein the first palladium surface further comprises a Volmer catalyst selected from the group consisting of platinum, iridium, rhodium, ruthenium, rhenium, and combinations thereof.
 11. The apparatus recited in claim 10, wherein the Volmer catalyst comprises a metal selected from platinum, iridium and combinations thereof.
 12. The apparatus recited in claim 10, wherein the ratio of the Volmer catalyst to the Tafel inhibitor on the first palladium surface is not greater than about
 5. 13. The apparatus recited in claim 10, wherein the ratio of the of the Volmer catalyst to the Tafel inhibitor on the first palladium surface is at least about 0.1.
 14. The apparatus recited in claim 1, wherein the second palladium surface comprises a Volmer catalyst selected from the group consisting of platinum, iridium, rhodium, ruthenium, rhenium, and combinations thereof.
 15. The apparatus recited in claim 1, wherein the electrolytic medium comprises a membrane disposed between the anode and the cathode.
 16. The apparatus recited in claim 15, wherein the membrane is a proton exchange membrane.
 17. The apparatus recited in claim 15, wherein the cathode compartment contains a first electrolyte and the anode compartment contains a second electrolyte, and wherein each of the first and second electrolytes are acidic electrolytes.
 18. The apparatus recited in claim 15, wherein the cathode compartment contains a first electrolyte and the anode compartment contains a second electrolyte, and wherein the first electrolyte is an alkaline electrolyte and the second electrolyte is an acidic electrolyte.
 19. The apparatus recited in claim 1, wherein the anode comprises platinum.
 20. The apparatus recited in claim 1, further comprising a power source operatively connected to the anode and to the cathode. 